How did the two towers usually cause trouble to the people in the boat?

To the reader, J. explains how easily tow-lines become tangled. On long journeys like this, it is common for travelers to take a break from rowing while someone tows the boat from shore. However, J. observes that the towers, on the shore, tend to become distracted by their conversation and stop paying attention to the boat. Whoever is left on the boat is usually uncomfortable or responsible for whatever crisis emerges, but is ignored by the towers.